A deoxyribonucleoprotein complex extracted from Simian Virus 40 infected monkey kidney cells in used as a model system for the study of chromatin structure and function. A pattern of nuclease-sensitive sites has been mapped to a unique region of the viral genome. Experiments are proposed to determine the location and nature of these sites and to map the SV40 genetic element(s) responsible for organization of this structure. Specifically, experiments are proposed to subfractionate SV40 chromatin, isolate the segment containing the region of interest, characterize histones and histone modifications as well as other proteins associated with this region. Additional experiments are proposed to reconstitute chromatin from isolated components and evaluate the recovery of the nuclease-sensitive pattern. SV40 mutants and physiological manipulations will be used to evaluate the significance of this unique chromatin structure.